The present invention is directed to a laser welding unit adapted to be mounted upon a robotic arm programmed to drive the unit along a seaming path to join, by an accurately focused laser beam, two sheet metal panel members to each other along a longitudinally extended seam.
Although useful in many other applications, the welding units of the present invention are particularly well adapted to use production line welding of sheet metal body panels of an automotive vehicle either in the welding of components to an individual panel or the assembly of major body panels on a body framing line. In a typical body framing operation, the main or basic sheet metal body panels are loosely assembled to each other upon a sled which conveys the panels to successive work stations. The basic panels include a floor panel, right and left body side panels, a fire wall, and header or cross frame members which extend transversely of the body between the side panels. The floor panel is fixedly mounted to the sled, and the other panels are loosely assembled on the floor panel as by a so-called toy tab arrangement.
This assembly is not a rigid assembly and, at the first welding station, a clamp system is employed to fixedly clamp the various panels in an accurately aligned relationship and to hold the panels in this position while the welding operation joining the panels to each other is performed. The conventional welding units employed are resistance welding units in which an electrode is forcibly pressed against one sheet metal panel at one side of the seam and energized to produce a spot weld.
This particular system has several drawbacks. Among these are the fact that the welding heads are quite bulky and clearance problems frequently arise in moving the welding head to and from its welding position past or around various portions of the vehicle body and the clamps and clamping frames employed to hold the body panels in position during the welding operation. The electrical power requirements of the welding operation are such that a relatively heavy power cable must be led to the head of the welding unit, and mechanism for applying fairly substantial pressures to press the welding electrode against the panels being welded are required.
Utilization of laser welding in applications such as automotive body framing has been actively investigated, however, power limitations and the problems involved in the design of a mirror system for guiding a laser beam from its generator to the operating end of a multi axis robotic arm have severely restricted the number of practical applications in which laser welding could be economically employed.
Recent developments in the laser field have resulted in units capable of increased power outputs and YAG lasers having power outputs fully sufficient to perform automotive body welding operations are now commercially available. These YAG units have the substantial advantage of enabling a laser beam of the required power to be conducted to a beam focusing unit of a welding head at the end of a robotic arm via a flexible fiberoptic cable, thus eliminating the requirement of a complex mirror system for transmitting the beam from the laser to the welding head.
In mass production laser welding operations, the precision with which the beam must be focused frequently greatly exceeds the degree of precision possible in positioning the sheet metal parts to be welded. In the automotive body welding panel application referred to above, manufacturing tolerances on the sled which carries the body panels, manufacturing tolerances of the clamps and clamping frame employed to clamp the panels in position for welding, dimensional variations of the individual panels from the ideal standard or unintended bending or warping of the panel, and the precision of alignment between the sled, clamping frame and welding unit all can introduce errors in positioning the panels relative to the laser beam focusing head which are many times greater than the maximum permissible variation of position relative to the point at which the laser beam is focused. Stated another way, commercially available robotic arms can be programmed to move a welding head along a path which is precisely located relative to a fixed reference frame. This path may follow a line which is matched to a seam line between two sheet metal members of a specific designed shape. The problem posed by welding such seams on a mass production basis is primarily that of accurately aligning the seaming line of the sheet metal parts with the path followed by the welding head on the robotic arm.
A second problem presented in laser welding two sheet metal parts to each other is the requirement that the two sheet metal parts be in abutment with each other or at worst within a minimum spacing from each other along the seaming line. For laser welding, the maximum spacing between the parts which will enable an adequate weld to be made is considered to be about 10% of the thickness of the thinner of the two sheet metal members. A three mil sheet metal thickness is quite common in automotive vehicle body applications. Stamped sheet metal panels cannot generally be formed with a degree of precision which would assure such spacing. In a resistance welding application, the two opposed panels are forcibly clamped against each other under relatively high pressure by the opposed electrodes, hence the spacing problem is of no concern in resistance welding applications. The laser beam, however, does not exert any physical force upon the parts which it is welding.
The present invention is directed to a laser welding unit addressed to the foregoing problems.